412 



MK. F. A. BATHER ON PETALOCRINUS. [Aug. 1 898, 



' British Fossil Crinoids — YIII : Cyathocrinus,^ Ann. Mag. Nat. Hist. 

 ser. 6, vol. ix (1892). 



In P. vishycensis the bilateral symmetry of the arm-fan is, as 

 shown by text-fig. 4, more subject to variation. This is due partly 

 to the fact that the bifurcations in each half do not always take 

 place at the same level, partly to the different number of branchings 

 that may occur iu each division. The latter difference may depend 

 to some extent on the different ages of the specimens ; but the re- 

 lation is not quite regular, since the levels at which the various 

 branchings take place vary slightly in different specimens. Text- 

 fig. 4 represents actual observed stages between the youngest known, 

 with 5 branches, and the oldest known with 22. This shows per- 

 fect obedience to the law already stated, which was in fact inferred 

 originally from this very diagram, the American specimens serving 

 only to confirm it. 



Pig. 4. — Diagram showing branching of grooves in arm-fan of 

 Petalocrinus vishycensis. 



semor ^ 

 U/t. \/i 





a.L. 



[The letters refer to the specimens, and the lines drawn across represent the 

 distal margin of each. Thus the diagram also shows the stages of growth. 

 Since the right and left halves correspond, except in P. vishycensis (senior), 

 the line is only drawn half way across.] 



A total of 28 finials was reached by P. inferior, and possibly by 

 P. vishycensis (senior). P. expansus and P. longus also have 28 

 and 27 finials respectively ; but in their case the high number is 

 attained, not by continuous regular dichotomy, but by a remarkable 

 meristic variation, which may be defined as the addition or inter- 

 calation of another half-fan. This is fully described under those 

 species (pp. 431 & 434). I cannot recall a similar variation among 

 Cyathocrinoidea, but certain crinoids in which the arms are pinnu- 

 late and normally bifurcate once, often have one ramus forking 

 again, so that the arm has 3 rami. This varies in individuals and 

 even in the rays of an individual.^ The unique type-specimens of 

 P. expansus and P. longus may therefore be abnormal arm-fans ; 

 but their specific independence depends on other characters than 

 this abnormalitv. 



^ A case of this kind was discussed in ' Brit. Foss. Crin.- 

 Ann. Mag. Kat. Hist. ser. 6, vol. vii (1891) pp. 404-405. 



-Y: Botryocrinus^ 



